Published Tuesday, December 7, 1999, in the San Jose Mercury News

Special to the Mercury News

Predicting fog proves a
cloudy proposition

Q.  How do forecasters predict fog?  Lucy Baldwin - Palo Alto

A.  Predicting the fog and low clouds of the Bay Area is certainly a non-trivial activity. Many meteorologists who have worked in other parts of the country say the combination of subtle fog patterns and microclimates makes it one of the trickiest phenomena to forecast.

To predict fog, we look at various factors. Among these is the amount of moisture in the lower atmosphere and whether the temperature will fall to the dew point, allowing condensation to occur. If low clouds are spreading inland from the coast, we also look at the depth of the marine layer and how low the
base of the clouds will be.

While some of this information is available from the computer simulations of the atmosphere, it often comes down to knowledge of the area and its fog patterns.

Q. Could you explain the ``green flash'' that we see in Hawaii at sunset? Of all the celestial events, this ranks right up there with the best.  Bob Hughes - San Jose

A. I must admit that I have not seen a ``green flash,'' but from all accounts, it is a wondrous sight.

When the sun sets, sometimes the last bit of light from the disk itself is an emerald green. The same is true of the first bit of light from the rising sun. The green flash is common and will be visible any time the sun rises or sets on a clear, unobstructed and low horizon.

It typically lasts one or two seconds, but on rare occasions much longer. You need a distinct horizon in the distance to see it. Consequently, it's often seen over the ocean, but can be seen over land if the horizon is far enough away.

Just before the sun rises, or just after it sets, you might see a brief flash of the color green. This is the result of looking at the sun through the great thickness of atmosphere. Water vapor in the atmosphere absorbs the yellow and orange colors of white sunlight, and it scatters the violet light. That leaves red and blue-green light to travel directly toward the observer.

Near the horizon, the sun's light is highly bent or refracted and it's as though there are two suns, a red one and a blue-green one, partly covering each other. The red one is always closest to the horizon, so when it sets or before it rises, you see only the fleeting edge of the blue-green disk.

Q.  If the moon is full here on the West Coast, what does it look like out in the Caribbean? The other night as I gazed at the gorgeous moon, I wondered if I will see that same moon a month from now on a Christmas cruise. Jan Foss - Los Gatos

A. The moon's phase is the same no matter where on Earth you are -- and a Caribbean cruise sounds like a great place.

A full moon occurs when the sun and moon are on opposite sides of Earth, and as Earth rotates through a day, their relative positions change only minutely. The last full moon of the year is Dec. 22.

Q. I would like to know how much rain we can expect this season. In which months will it rain? Ashwini Nawathe -

A. It's impossible to predict precisely how much rain the upcoming season will produce, let alone for any given month. However, because La Niņa continues in the equatorial Pacific, a good first estimate is to look at previous seasons when there has been a La Niņa.

This includes last year, which was typical of previous La Niņas, with the Bay Area receiving about average rainfall while areas to the north were above the average and areas to the south were below the norm.

Jan Null, a certified consulting meteorologist and owner of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send him questions c/o Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone and fax them at (510) 657-2246 or e-mail them to

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